3.15 Article Backlog

If you have a slow connection, but the idea of using caching seems
unappealing to you (and it is, really), you can help the situation some
by switching on the backlog. This is where Gnus will buffer
already read articles so that it doesn’t have to re-fetch articles
you’ve already read. This only helps if you are in the habit of
re-selecting articles you’ve recently read, of course. If you never do
that, turning the backlog on will slow Gnus down a little bit, and
increase memory usage some.

If you set gnus-keep-backlog to a number n, Gnus will store
at most n old articles in a buffer for later re-fetching. If this
variable is non-nil and is not a number, Gnus will store
all read articles, which means that your Emacs will grow without
bound before exploding and taking your machine down with you. I put
that in there just to keep y’all on your toes.